CHICAGO Syngenta AG is pressing ahead with U.S. sales of a new corn trait that is not approved in China, fueling concerns of another jarring GMO trade quarrel with the world's fastest-growing food importer.

While the U.S. grain industry lobbies the Chinese government to stop rejecting cargoes of corn containing another unauthorized genetically modified Syngenta strain, farmers in the Midwest are weighing whether to take a chance on the Swiss-based company's new product, engineered to combat pests called rootworms.

Syngenta, the world's largest crop chemicals company, said its Agrisure Duracade trait will be available for planting for the first time this year in "limited quantities" after U.S. authorities cleared it for sale and cultivation last year.

For farmers it may be a risk worth taking: Infestations of rootworms have developed into a major challenge in top-producing states such as Iowa and Illinois, because farmers have increasingly been planting corn on the same fields year after year to take advantage of high prices. The traditional practice of rotating corn with soybeans had helped better control pests.

But the planting of Duracade threatens new disruptions - and millions of dollars in losses - for global grain traders if the strain gets mixed into the mainstream supply chain and prompts another round of rejections from China, as some analysts fear.

The commercialization of Duracade could "further contaminate, slow down, gum up" shipments of U.S. corn to China, Rich Feltes, vice president of research for commodity brokerage R.J. O'Brien, said in a recent interview.

The issue has already split the farm sector, as a national growers' group urged the U.S. Agriculture Department to approve the strain while grain lobbyists warned in 2012 that the trait could hurt U.S. trade because it was not accepted by all major importers.

Some U.S. seed companies have declined to license the new Duracade trait from Syngenta to insert into seeds, out of concern that growers could be stuck if grain elevators and exporters refuse to buy corn that contains it.

The United States is expected to export 1.45 billion bushels of corn in the marketing year that ends August 31, accounting for 10 percent of the last harvest.

VIPTERA STANDOFF

In November, Chinese authorities began rejecting some U.S. corn cargoes that contained Syngenta's Agrisure Viptera variety, known as MIR 162, which has been awaiting Beijing's acceptance for more than two years.

Why some imports have been allowed while others have been stopped is not clear, and analysts and traders have offered various explanations, including possible trade tensions or a desire to support domestic corn prices.

Whatever the cause, a total of 600,000 ton of U.S. corn and corn products have been denied entry, weakening U.S. prices and providing the latest example of GMO use disrupting agricultural trade.

Syngenta is not considering any steps to pull back Duracade from the market because it believes growers need access to new technologies, spokesman Paul Minehart said. The number of acres planted with seed containing the trait will be determined by seed production volumes and shipping time lines, he said.

In line with industry standards, Minehart said, Syngenta commercializes corn traits once it has approval from countries with "functioning regulatory systems."

"The regulatory systems in China and the EU are not considered functioning according to industry policy, meaning their decision-making processes are not predictable, are not completed in a timely manner and can be subject to political influence," he said.

While the European Union approved Viptera for import in 2012, it has not yet approved Duracade for import.

Syngenta applied for Chinese import approval of Duracade after U.S. authorities cleared the trait in February 2013. In theory, China's agriculture ministry has 270 days to make a decision. However, industry sources say it can take as long as two years after a strain is approved by the United States.

Syngenta won import approval for the Duracade trait from Mexico and South Korea in September and from Japan in August. It also has import approval from Australia, New Zealand and Taiwan.

"United States and China do not have synchronous approval process. If China does not change the current approval procedure, it remains an issue in future U.S. exports to China," said Li Qiang, senior analyst with Shanghai-based JC Intelligence Co Ltd, a private agriculture-consulting firm.

Until recent years, China's slower system for approving new GMO products was less of an issue, since it imported almost no U.S. corn. But shipments have surged in recent years, accounting for almost 14 percent of U.S. corn exports in 2011/12.

The increase has made it increasingly perilous for the trading industry to have China excluded from the list of countries whose approval is required to launch a new trait, some say. China mainly buys U.S. corn to feed livestock.

"China may not have a functioning approval system, but they represent the largest growth potential in U.S. corn exports," Feltes said. "Let's get real."

"TICKING TIME BOMB"

John Latham, president of Iowa-based Latham Hi-Tech Seeds, has licensed the Duracade trait from Syngenta for this spring. But without approval from China, he says, U.S. corn growers have been wary of booking those seeds and that some elevators may refuse to accept corn with the trait.

Peterson Farms Seed in North Dakota decided against licensing the Duracade trait from Syngenta because of concerns about China, agronomy manager Adam Spelhaug said. The firm sells corn with the Viptera trait, he said, while making sure that customers who buy those seeds can sell to markets that accept the trait.

"Unless it has Chinese approval, we're probably not going to bring something else into our bag right now," he said.

Rootworms are a "ticking time bomb" in U.S. farmers' fields that are estimated to cost more than $1 billion annually in yield losses and treatment costs, Syngenta said in an online ad for the trait. Duracade "delivers unmatched corn rootworm control" that is distinct from another rootworm-fighting trait previously released by Syngenta, the company said.

Rival Monsanto Co, the world's largest seed company, introduced corn rootworm protected products in 2003. However, a group of academic corn experts in 2012 warned the U.S. Environmental Protection Agency of growing rootworm resistance to GMO corn.

With Duracade coming to market, "the probability of overall of resistance goes down because there's another tool that can be used" to fight rootworm, said Rodney Weinzierl, director of the Illinois Corn Growers Association.

The National Corn Growers Association in 2012 told the USDA that it supported Duracade because of rootworm threats. A spokesman did not respond to requests for an updated comment.

PREVIOUS WARNINGS

Even if corn containing Duracade is planted on a small number of acres, it could accidentally be shipped to China, exporters said. Varieties are often mixed with each other because they are grown in fields near each other and harvested, transported and stored together.

In 2011, Bunge North America, a unit of agricultural trading house Bunge Ltd, refused to accept Viptera because it had not been approved by major export destinations. Archer Daniels Midland Co and Cargill Inc that year declined to accept at elevators grain that was not approved for commercial use in the EU, which included Viptera, without prior written notice.

Representatives of Bunge, Cargill and ADM declined to comment or did not respond to requests for comment on Duracade. Two top U.S. grain groups - the National Grain and Feed Association and North American Export Grain Association -- had discouraged the USDA from clearing Duracade.

The lack of import approvals for new traits creates "a risk of significant economic losses to U.S. grain and oilseed producers and markets," the groups said in a 2012 letter. Chinese quarantine officials can reject shipments that contain any amount of an unauthorized strain, they said. The groups declined to provide updated comments on the matter.

Corn importers in China typically stipulate in their contracts that the seller bears all costs related to possible GMO risks, industry sources say.

The USDA in 2012 responded to concerns about missing import approvals by saying that grain elevators could refuse to buy corn containing Duracade. Commercialization of the trait could even enhance trading "through more efficient production of corn supplies worldwide," the USDA said at the time.

(Additional reporting by Carey Gillam in Kansas City and Niu Shuping in Beijing; Editing by Jonathan Leff and Steve Orlofsky)

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